I installed Firebug but I don't see the icon. Where is it?
If you use Firefox 3.6 or earlier you should see the Firebug icon in the status bar. Starting from Firefox 4.0 the Firebug Start Button moved to the toolbar. If it's not visible there, it can be found inside the Customize Toolbar dialog. For more info about this please read Honza's blog post.

Are there options, which you can't set through the UI?

Console Panel

How does the Console panel work?

Does Firebug interfere in any way with jQuery's $ shortcut?

Although Firebug uses the '$' notation it doesn't interfere with your code. Sometimes you may have issues testing code in the console but can easily avoid them by using the jQuery's alternative notation:

jQuery('div');

Instead of:

$('div');

Why do I get the error "console is not defined"?

On Firefox versions up to 3.6 you will see this message in the Firefox Error Console as long as the Console Panel is not enabled. When the Console panel is enabled, it makes the 'console' object available to the page's javascript.
Since Firefox 4.0 the 'console' object is already defined through the Firefox internal developer tools. So this message should not appear anymore.

HTML Panel

How does the HTML panel work?

Why keeps the HTML panel scrolling up?

If the HTML panel scrolls up by itself, you probably have a timed event inside your code, which inserts or changes HTML and triggers the panel to scroll to this change. To avoid scrolling to the change uncheck Expand Changes and Scroll Changes Into View.

Why do the HTML panel contents look different than my source file?

The HTML panel shows a live view on what the browser is showing. I.e. if a JavaScript makes changes to the HTML, the HTML panel will display these changes. Also it displays the HTML structure formatted as tree structure. So it doesn't matter if the code coming from the server is minified or has thousands of spaces in between the tags, the display inside the panel will be the same.

How can I save changes I made inside the HTML panel back to my source file?

You can't. As John J. Barton wrote on the newsgroup:

Editing in Firebug is kinda like taking out the pickles from and
adding mustard to a restaurant sandwich: you can enjoy the result, but
the next customer at the restaurant will still get pickles and no
mustard.

The reason is that the HTML output is normally dynamically generated by some kind of server script language like e.g. PHP.

You get an error message inside Firefox' Error Console like "installLocation has no properties" => Try a new profile; Re-install Firefox.

If you're using Windows, you can try the following:

With Firefox closed, locate your profile directory, delete these temporary files, they will be automatically re-created by Firefox:extensions.iniextensions.cacheextensions.rdfcompreg.dat

Disable any Antivirus or Backup program you may have running and then install Firebug.It may be that some software is stopping Firefox creating the file structures needed for the installation process.

Run Firefox as Administrator. It may be the same as in reason 2, because firefox doesn't have enough permissions.

Manually install Firebug.This can be done easily, just unzip the XPI (it's a normal zipped file with the extension changed) in your extensions folder, inside your profile. You should unzip firebug's XPI content inside a folder named 'firebug@software.joehewitt.com'. Do this with Firefox closed.

To get the XPI go to Firebug on addons.mozilla.org (AMO), right click on the "Add to Firefox" button and select Save Link As....

If your problem didn't get solved or your error message is not listed above:

Firebug crashes Firefox. What can I do?

Crash means the program exits.

Note that we're not responsible for fixing Firefox crashes - even when you can confirm that they are related to Firebug.
Browser crashes can have different reasons. Generally in such cases it's a browser API Firebug uses, which is corrupt.
When the browser crashes the Mozilla Crash Reporter should appear. To get the crash fixed please report it to Mozilla attaching the crash report URL, so they can fix it. Please inform us about the crash and post us the link to the report via the discussion group or IRC, though. Other people might have the same problem. In these cases we could point them to the report.
If you see a crash, which affects many people, we might be able to find a workaround in some cases until it's fixed in Firefox.

Firebug hangs itself or Firefox. Why?

Hang means the program is still running (it did not crash) but it fails to respond to user commands. The causes and diagnosis is completely different from a crash.
You should follow the steps for what to do when you experience a bug, so you can find out if it's a problem in Firebug or the browser and create a test case.